Protein kinases constitute a large family of structurally related enzymes that are responsible for the control of a variety of signal transduction processes within the cell. Protein kinases, containing a similar 250-300 amino acid catalytic domain, catalyze the phosphorylation of target protein substrates.
The kinases may be categorized into families by the substrates in the phosphorylate (e.g., protein-tyrosine, protein-serine/threonine, lipids, etc.). Tyrosine phosphorylation is a central event in the regulation of a variety of biological processes such as cell proliferation, migration, differentiation and survival. Several families of receptor and non-receptor tyrosine kinases control these events by catalyzing the transfer of phosphate from ATP to a tyrosine residue of specific cell protein targets. Sequence motifs have been identified that generally correspond to each of these kinase families [Hanks et al., FASEB J., (1995), 9, 576-596; Knighton et al., Science, (1991), 253, 407-414; Garcia-Bustos et al., EMBO J., (1994), 13:2352-2361). Examples of kinases in the protein kinase family include, without limitation, abl, Akt, bcr-abl, Blk, Brk, Btk, c-kit, c-Met, c-src, c-fms, CDK1, CDK2, CDK3, CDK4, CDK5, CDK6, CDK7, CDK8, CDK9, CDK10, cRaf1, CSF1R, CSK, EGFR, ErbB2, ErbB3, ErbB4, Erk, Fak, fes, FGFR1, FGFR2, FGFR3, FGFR4, FGFR5, Fgr, flt-1, Fps, Frk, Fyn, Hck, IGF-1R, INS-R, Jak, KDR, Lck, Lyn, MEK, p38, PDGFR, PIK, PKC, PYK2, ros, Tie, Tie-2, TRK, Yes, and Zap70.
Studies indicated that protein kinases play a central role in the regulation and maintenance of a wide variety of cellular processes and cellular function. For example, kinase activity acts as molecular switches regulating cell proliferation, activation, and/or differentiation. Uncontrolled or excessive kinase activity has been observed in many disease states including benign and malignant proliferation disorders as well as diseases resulting from inappropriate activation of the immune system (autoimmune disorders), allograft rejection, and graft vs host disease.
It is reported that many diseases are associated with abnormal cellular responses triggered by protein kinase-mediated events. These diseases include autoimmune diseases, inflammatory diseases, bone diseases, metabolic diseases, neurological and neurodegenerative diseases, cancer, cardiovascular diseases, allergies and asthma, Alzheimer's disease and hormone-related diseases. In addition, endothelial cell specific receptor PTKs, such as VEGF-2 and Tie-2, mediate the angiogenic process and are involved in supporting the progression of cancers and other diseases involving uncontrolled vascularization. Accordingly, there has been a substantial effort in medicinal chemistry to find protein kinase inhibitors that are effective as therapeutic agents.
Many cancers are characterized by disruptions in cellular signaling pathways that lead to uncontrolled growth and proliferation of cancerous cells. Receptor tyrosine kinases (RTKs) play a crucial role in these signaling pathways, transmitting extracellular molecular signals into cytoplasm and/or nucleus of a cell. RTKs are transmembrane proteins that generally include an extracellular ligand-binding domain, a membrane-spanning domain and a catalytic cytoplasmic tyrosine kinase domain. The binding of ligand to the extracellular portion is believed to promote dimerization, resulting in trans-phosphorylation and activation of the intracellular tyrosine kinase domain (Schlessinger et al. Neuron 1992; 9:383-391).
Considering the lack of currently available treatment options for the majority of the conditions associated with protein kinases, there is still a great need for new therapeutic agents that inhibit these protein targets.